1. Field of the Invention
The present invention relates to an image pickup apparatus, such as a digital camera or a video camcorder (video camera recorder), a method of controlling the image pickup apparatus, and a computer-readable nonvolatile storage medium storing a computer-executable program implementing the method, and more particularly to signal processing for correcting image quality degradation due to a lens unit.
2. Description of the Related Art
A lens unit for forming an object image on an image pickup element involves various factors which cause image degradation, such as distortion, blur, or peripheral light amount drop, on the object image.
For example, magnification chromatic aberration which blurs the peripheral color of an image is caused when red, green, and blue lights having passed through the lens unit form images at respective different locations in a direction orthogonal to the optical axis according to the respective wavelengths of the color lights. A peripheral light amount drop as a phenomenon that the amount of light becomes smaller toward the periphery of an image than in the central portion of the image is caused e.g. by so-called vignetting which occurs due to mechanical vignetting by the barrels of the lens unit or the cosine fourth law.
For example, the magnification chromatic aberration as one of the image quality degradations due to a lens unit can be reduced to some extent by using a combination of a plurality of lenses having complicated shapes or by using fluorite which rarely causes chromatic dispersion, as a lens material. The peripheral light amount drop can also be reduced to some extent by using a lens having a large image circle. In either of the above-mentioned cases, however, it is impossible to fully prevent image quality degradation.
To solve this problem, there have been proposed techniques in which image quality degradation due to the characteristics of a lens unit is reduced by image correction performed by signal processing.
For example, a technique has been disclosed in which correction data on peripheral light amount drops are stored in a ROM in association with aperture values, focal lengths, or shooting distances of a lens unit, and a digital signal processor corrects an image by carrying out signal processing according to an actual aperture value, actual focal length, or actual shooting distance of the lens unit (see Japanese Patent Laid-Open Publication No. 2003-110936).
Further, there has been disclosed another technique in which image quality degradation due to lens aberration is corrected by signal processing based on aberration information on a lens unit, object luminance data, and color difference data (see Japanese Patent Laid-Open Publication No. 2000-3437).
The above-mentioned techniques of correcting image quality degradation due to a lens unit by carrying out signal processing are applied not only to digital cameras, but also to digital video camcorders and cameras equipped with a so-called live view (LV) function for displaying images picked up by an image pickup element, on a real-time basis. In other words, image quality degradation of a moving image due to a lens unit is corrected in real time according to the optical state of the lens unit.
For example, a technique has been disclosed in which correction values for use in correcting aberration, the amount of peripheral light, and so forth are stored in advance in a memory within an interchangeable lens unit, and when the power of an image pickup apparatus is turned on or when the interchangeable lens unit is connected to the image pickup apparatus, the information of the correction values is sent from the memory within the interchangeable lens unit to the image pickup apparatus in advance. Then, while receiving information on the optical state of the interchangeable lens unit in real time, the image pickup apparatus performs image correction using corresponding ones of the correction values received in advance according to the optical state of the interchangeable lens unit (see Japanese Patent Laid-Open Publication No. 2008-96907).
However, a correction value for use in correction of a peripheral light amount drop varies according to the aperture value and the focus position even in a single lens unit. Further, in a lens unit having a zoom mechanism, the correction value varies according to the focal length as well. Therefore, as the lens unit has more optical statuses, the number of correction values for correction of peripheral light amount drops becomes enormous, which not only makes it necessary to provide a larger memory area necessitated, but also takes longer communication time for data reception.
To solve this problem, there has been disclosed a technique in which a peripheral light amount drop correction value is not provided for each zoom position, each aperture value, or each focus position, but a correction value corresponding to the current zoom position, the current aperture value, or the current focus position is calculated by determining and interpolating from discrete correction values (see Japanese Patent Laid-Open Publication No. 2006-121384).
When an image pickup apparatus grasps an optical status of the lens unit and then starts calculating a correction value suitable for the optical status of the lens unit, correction is delayed at least by the time taken for the calculation of the correction value. During “delay” the time period, a correction value obtained in the immediately preceding loop is used for correction of image quality degradation. As a consequence, e.g. in the correction of a peripheral light amount drop, an insufficiently corrected state can occur in which luminance on the periphery of an image is reduced due to the use of the correction value obtained in the immediately preceding loop, because the correction value is smaller than an appropriate value. Otherwise, an overcorrected state can occur in which luminance on the periphery of an image is increased because the correction value is larger than an appropriate value. As long as the optical state of the lens unit is being changed, the way an object image looks also changes, and hence insufficient correction or overcorrection is not conspicuous on the image, but immediately after the changing of the optical state of the lens unit is stopped, the insufficient correction or overcorrection becomes conspicuous.
To make the correction delay time during which the correction is delayed as short as possible, it can be envisaged that immediately after calculation of a new correction value is completed, the image is corrected using the new correction value. Assuming, by way of example, that an image is insufficiently corrected due to correction delay, when the insufficiently corrected image is changed to an appropriately corrected image, the periphery of the image, which appeared dark to a user, comes to look bright. At this time, if the amount of change in the optical state of the lens unit is large, the amount of increase in the luminance level of the periphery of the image also becomes large, making the change in the luminance level conspicuous. As a consequence, the user perceives the change as flickering of the image, and hence is given a feeling of wrongness.
To solve this problem, a method can be envisaged in which even after the completion of calculation of a new correction value, the original correction value is gradually shifted to the new correction value instead of correcting an image using the new correction value immediately. However, assuming that an image is overcorrected due to correction delay, the image cannot be promptly corrected from the overcorrection state even though the calculation of a new correction value is completed. Since an overcorrected image is more conspicuous than an insufficiently corrected image, if the overcorrection state of the image is maintained or left uncorrected, the user is given a feeling of wrongness.
A similar problem occurs in correction of a distortion aberration or a chromatic aberration. For example, in distortion aberration correction, which is performed to correct distortion of the whole shape of an image, an area of the image which existed before the correction is excluded from the image having undergone the distortion aberration correction. For this reason, the range of an object included in an image differs depending on the amount of distortion aberration correction. Therefore, when the amount of distortion aberration correction changes sharply, a change in the object range included in the angle of view becomes conspicuous, which gives a feeling of wrongness to a user. Further, when overcorrection occurs in the distortion aberration correction, the image is distorted into a more unnatural shape, and when this unnaturally distorted state of the image is maintained or left uncorrected, the user is given a feeling of wrongness.